"All but one gunshot wound were to the lower extremities," Graham said. "The one fatal wound was to the head."

Police said the teen fired a pistol three times at an off-duty St. Louis officer who was wearing his uniform while moonlighting for a security company. The officer fired back 17 times, authorities said.

"It's a clear case of this young man being gunned down by an insensitive white officer who was off-duty," Akbar Muhammad, who also is a member of a coalition calling for the officer who shot Brown to be prosecuted, told CNN affiliate KTVI.

Neil Bruntrager, a lawyer for the St. Louis Police Officers' Association, dismissed accusations that 17 shots were excessive.

"When he uses deadly force, he uses deadly force until the threat is gone," he said on "Erin Burnett OutFront."

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Without video of the incident or unbiased witnesses, the police and the public must rely on physical evidence, he said. A 9 mm pistol allegedly fired by Myers was found at the scene, police said. It was jammed, which shows that it had been fired, Bruntrager said.

But Van Jones, a civil rights activist and CNN contributor, said there's a lot of police mistrust after the Brown killing.

"You're seeing a wholesale breakdown of trust," he said.

The officer, a six-year veteran who has not been named, was placed on administrative leave. He served two tours of duty with the military in Iraq and was awarded a Bronze Star and a Purple Heart, Bruntrager said.

While on patrol, the officer saw three men run away as he approached, Dotson said. He chased them after one held up his pants in a way that made the officer believe he may be carrying a gun, Dotson said.

The officer tussled with one teen, who ran off, then turned around and fired at least three shots toward him, Dotson said.

The officer returned fire, killing the teen.

Additional marches are planned over the weekend around the St. Louis region, organizers said.